Photoconductivity spectra of single-carbon nanotubes: Implications on the nature of their excited states

We have measured the photoconductivity excitation spectra of individual semiconducting carbon nanotubes incorporated as the channel of field-effect transistors. In addition to the pronounced resonance that correlates with the second van Hove transition (E(22)) in semiconducting carbon nanotubes, a weaker sideband at about 200 meV higher energy is observed. Electronic structure calculations that include electronphonon coupling indicate that the spectra originate from the simultaneous excitation of an exciton (main resonance) and a C-C bond stretching phonon (sideband). The spectral features are not compatible with an interband interpretation of the excitation involved.